The present invention is directed to the field of materials fabrication, and in particular to the preparation of nanometer-sized particles that are intentionally doped with a low concentration of manganese (Mn) impurities. These materials have applications, among others, as phosphors, laser materials, fluorescent labels for biological assaying, magnetic memory, and in xe2x80x9cspintronics.xe2x80x9d
As a result of research over the last ten years, it is now possible to synthesize extremely high-quality colloidal nanometer-scale particles from a wide variety of materials. xe2x80x9cNanometer-scalexe2x80x9d as used herein refers to particles that have diameters on a scale of approximately 10xe2x88x929 meters (1 nanometer (nm) equals 1xc3x9710xe2x88x929 meters). xe2x80x9cColloidalxe2x80x9d refers to particles that after preparation are easily dispersed in a suitable solvent or carrier material. For many materials it is possible to synthesize a specific desired particle size in the nanometer-scale regime. This ability is important since many of the properties of these particles, also called xe2x80x9cnanocrystals,xe2x80x9d are strongly dependent on the exact size of the particle.
The first method to produce high-quality semiconductor nanocrystals was that of C. Murray et al. discussed in xe2x80x9cSynthesis and Characterization of Nearly Monodisperse CdE (E=S, Se, Te) Semiconductor Nanocrystallites,xe2x80x9d Journal of the American Chemical Society, Vol. 115, pp. 8706-8715 (1993), which demonstrates how to obtain cadmium selenide (CdSe) nanocrystals that are extremely uniform in terms of their size, shape, structure, surface passivation and composition. With this method one can easily prepare CdSe particles with any desired size between 1.2 and 11.5 nm with size distributions less than 5% (rms deviation). This approach is of general use and has been extended to other materials, such as zinc selenide (ZnSe), for example. See M. Hines et al., xe2x80x9cBright UV-Blue Luminescent Colloidal ZnSe Nanocrystals,xe2x80x9d Journal of Physical Chemistry B, Vol. 102, No. 19, pp. 3655-3657 (1998). This broader class of high-quality semiconductor nanocrystals is referred to herein as nanocrystals made by the xe2x80x9corganometallic approachxe2x80x9d. Since colloidal semiconductor nanocrystals made by the organometallic approach can be highly fluorescent and robust to repeated excitation by light, application of these materials includes use as laser materials as disclosed in U.S. Pat. No. 5,260,957 to Hakimi et al., and as a fluorescent tag (or marker) in biological applications such as biological assaying as disclosed in U.S. Pat. No. 5,990,479 to Weiss et al.
The interesting properties of these materials have encouraged researchers to go beyond pure nanocrystals and to investigate crystallites that are intentionally doped with impurities; that is, nanocrystals in which a small number of the atoms have been intentionally replaced by another element. This is motivated, in part, by the fact that most of the interesting properties of bulk semiconductor materials result from dopants. In the research of doped nanocrystals, much effort has focused on II-VI semiconductor nanocrystals, such as ZnS or CdS, which are doped with manganese (Mn). See, e.g., R. Bhargava et al., xe2x80x9cOptical Properties of Manganese-Doped Nanocrystals of ZnS,xe2x80x9d Physical Review Letters, Vol. 72, No. 3, pp. 416-419 (1994); D. Gallagher et al., xe2x80x9cHomogeneous Precipitation of Doped Zinc Sulfide Nanocrystals for Photonic Applications,xe2x80x9d Journal of Materials Research, Vol. 10, No. 4, pp. 870-876 (1995). Initially, this choice was driven by the analogous bulk materials (i.e. bulk semiconductor crystals doped with Mn impurities), referred to as dilute magnetic semiconductors (DMS) or semimagnetic semiconductors. Due to the sp-d exchange interaction between the semiconductor and the Mn, bulk DMS crystals have interesting magnetic and magneto-optical properties. See J. Furdyna, xe2x80x9cDiluted Magnetic Semiconductors,xe2x80x9d Journal of Applied Physics, Vol. 64, No. 4, pp. R29-R64, (1988). DMS nanocrystals should exhibit even more exotic behavior since spinxe2x80x94spin exchange interactions are enhanced by the confinement of the electron and hole. However, more recently, an additional motivation has been recognized. DMS nanocrystals have potential applications in xe2x80x9cspintronicsxe2x80x9d, electronic devices where not only the charge of the electron, but also its spin, is utilized. See D. Awschalom et al., xe2x80x9cElectron Spin and Optical Coherence in Semiconductors,xe2x80x9d Physics Today, Vol. 52, pp. 33-38 (Jun. 1999).
In the prior art, Mn-doped ZnS nanocrystals and a method for preparing same have been discussed, as disclosed in U.S. Pat. No. 6,048,616 to Gallagher et al. (hereinafter xe2x80x9cthe ""616 patentxe2x80x9d). However, this prior invention, which does not use the organometallic approach, yields nanocrystals that are not of high quality (i.e. they are not uniform in size, shape, and structure). In addition, the method of this prior invention is to prepare the nanocrystals at room temperature, an approach that is known to produce many defects in the nanocrystals. These nonuniformities and defects limit the usefulness of the nanocrystals. For example, deviations in the size of the particles will make their properties highly inhomogeneous. In the ideal doped sample, all the nanocrystals would be exactly the same and their properties would be identical. If one could obtain Mn-doped nanocrystals with a method similar to the organometallic approach, these particles would have much better properties than those of the ""616 patent, for example.
Furthermore, undoped nanocrystals made by the organometallic approach can be induced to form close-packed solids, in which the nanocrystals are in contact but have not fused. See C. Murray et al., xe2x80x9cSelf-Organization of CdSe Nanocrystallites into Three-Dimensional Quantum Dot Superlatticesxe2x80x9d, Science, Vol. 270, pp. 1335-1338 (1995). Close-packed nanocrystals, referred to as xe2x80x9cquantum-dot solids,xe2x80x9d are novel artificial materials in which both the properties of the individual nanocrystalline building block and the interaction between them can be controlled. In these materials, the behavior of the solid can be tailored to fit a specific need.
If Mn-doped nanocrystals could be obtained with a method similar to the organometallic approach, new quantum-dot solids could be fabricated wherein each of the nanocrystalline building blocks has additional properties due to the impurity. However, due to the poor quality of Mn-doped ZnS particles such as those obtained by the method of the ""616 patent, such quantum dot solids have not been made in the prior art.
Therefore, to obtain high-quality Mn-doped semiconductor nanocrystals, it would be useful to devise a new method, based upon the general organometallic approach, that could incorporate Mn into the nanocrystal. To date, only one attempt has been reported in the prior art. See F. Mikulec et al., xe2x80x9cOrganometallic Synthesis and Spectroscopic Characterization of Manganese-Doped CdSe Nanocrystals,xe2x80x9d Journal of the American Chemical Society, Vol. 122, pp. 2532-2540 (2000). However, this work, which describes an extensive multi-year effort to synthesize Mn-doped CdSe nanocrystals, concludes that the Mn is not incorporated inside the nanocrystal, but rather tends to attach to the surface of the nanocrystal. Since many of the properties and applications of Mn-doped nanocrystals depend on the Mn impurity being inside the nanocrystal, this work has not succeeded in preparing useful Mn-doped semiconductor nanocrystals. Indeed, the lack of success of such an extensive effort has indicated thus far that the general organometallic approach may not be useful for Mn-doping.
Accordingly, a need exists to provide a method for producing Mn-doped nanocrystals which incorporates the organometallic approach, incorporates the Mn inside the nanocrystal, and thereby provides nanocrystals which are more uniform in size, shape, surface passivation, composition and crystallinity than in the prior art, and in which the size of the nanocrystals and the concentration of the impurities can easily and desirably be controlled.
Generally speaking, in accordance with the invention, a method for manufacturing high-quality Mn-doped nanocrystals is provided. The method generally comprises the steps of: (a) combining an organometallic manganese precursor with an organometallic Group II precursor and an organometallic Group VI precursor to provide a precursor mixture; (b) diluting the precursor mixture with a dilution solvent to provide an injection mixture; (c) heating a coordinating solvent; (d) stirring the heated coordinating solvent; and (e) injecting the injection mixture into the heated coordinating solvent while the heated coordinating solvent is being stirred.
Nanocrystals obtained by the method of the present invention are much more uniform in size, shape, and crystallinity than in the prior art. The size of the nanocrystals and the concentration of the impurities can also be easily controlled. Furthermore, these particles are highly luminescent. For example, by exciting ZnSe nanocrystals manufactured in accordance with the present invention with light (e.g. with a galium nitride (GaN) laser at wavelengths less than xcx9c410 nm or a frequency-tripled Nd:YAG laser at xcx9c355 nm), very efficient emission is obtained from the Mn impurity. Since this emission (at 585 nm) is strongly red-shifted from the excitation wavelength, the nanocrystals produced by the present invention are particularly useful in biological assaying.
In a preferred embodiment of the invention, high-quality, Mn-doped ZnSe nanocrystals are prepared. In another embodiment, high-quality, Mn-doped zinc sulfide (ZnS) nanocrystals are prepared. In a further embodiment, high-quality, Mn-doped zinc telluride (ZnTe) nanocrystals are prepared.